This application is based on and claims Convention priority to Japanese patent application No. 2007-199218, filed Jul. 31, 2007, the entire disclosure of which is herein incorporated by reference as a part of this application.
The present invention relates to a sensor equipped wheel support bearing assembly having a load sensor built therein for detecting a load imposed on a bearing unit of a vehicle wheel.
As a technique for detecting a load imposed on each of vehicle wheels of an automotive vehicle, a sensor equipped wheel support bearing assembly capable of detecting the load by detecting a strain induced in an outer diametric surface of an outer ring flange of the wheel support bearing assembly. (See, for example, the Patent Document 1 listed below.) A wheel support bearing assembly has also been suggested, in which a strain amplifying mechanism in the form of an L-shaped member is fitted over a flange portion and an outer diametric portion of a stationary ring and a strain gauge is pasted to a portion of such strain amplifying mechanism. (See, for example, the Patent Document 2 listed below.)    [Patent Document 1] JP Laid-open Patent Publication No. 2002-098138    [Patent Document 2] JP Laid-open Patent Publication No. 2006-077807
According to the technique disclosed in the Patent Document 1 listed above, a strain generated as a result of deformation of a flange portion of a stationary ring is detected. However, such a problem has been found that since the deformation of the flange portion of the stationary ring is accompanied by a slippage occurring between a flange surface and a knuckle surface in the event that it exceeds a stationary frictional force, application of a repeated load results in generation of a hysteresis in an output signal.
By way of example, in the event that the load acting in a certain direction relative to the wheel support bearing assembly becomes large, no slippage occur between the stationary ring flange surface and the knuckle surface because the stationary frictional force is initially higher than the load, but when it exceeds a certain quantity enough to overcome the stationary frictional force, the slippage occurs therebetween. If the load is reduced while in that condition, no slippage occur at first due to the stationary frictional force, but when it attains a certain magnitude, the slippage occurs. As a result, if an attempt to calculate the load is made at such a portion where the deformation occurs, such a hysteresis as shown in FIG. 12 occurs in the output signal.
Also, even in the technique disclosed in the Patent Document 2 listed above, a site at which the strain amplifying mechanism in the form of the L-shaped member is fixed to the flange surface, tends to be affected by a friction (slippage) occurring between the flange surface and the knuckle surface and, therefore, a problem similar to that discussed above is found therein.
Furthermore, where a load Fz acting on the wheel support bearing assembly in the vertical direction is to be detected, the amount of strain is small since the amount of a stationary ring deformation relative to the load Fz is small, and, accordingly, the above described techniques are incapable of detecting the load Fz with a high accuracy because of the detecting sensitivity being low.